Numerous techniques, particularly in the medical field, to improve not only diagnostics, as MRI systems achieved, but to use magnetics in proactive disease intervention and cure are being studied. Several proposed techniques involve magnetic propulsion of magnetic objects through the bloodstream or other anatomical structures. For such applications, sets of electromagnet type coils that can generate three axis orthogonal gradient magnetic fields are useful to propel the magnetic objects in various directions. High magnetic propulsion forces can be generated by electromagnet type coils that generate high magnetic gradients. Superconducting gradient coils (SGC) are of interest because they can produce higher gradient field strength, therefore higher propulsion forces, than are practically possible using copper coils. Typically, sets of electromagnet type coils that generate gradient magnetic fields are called gradient coils. Gradient magnetic fields are commonly called gradient fields. Three axis gradient coils are used in Magnetic Resonance Imaging (MRI) scanners, as well as Nuclear Magnetic Resonance (NMR) spectrometers. Higher gradient fields are useful to some applications including diffusion weighted imaging and high resolution imaging.
An electromagnet type coil that uses superconducting wire, or cable of superconductive wires, is called a superconducting coil. Superconducting wires transport electric current without resistance. A superconducting coil or magnet may be wound with unitary wire or with a cable containing superconducting wires (either are herein denoted superconducting conductors). A DC magnet that uses a superconducting conductor produces no heat so long as the magnet is kept below its critical temperature, TC. However, since in many applications gradient coils are pulsed (i.e. they are charged by alternating current (AC)), the superconducting conductors making up an AC superconducting magnet generate significant heat as the result of so-called AC losses. This heat if unmanaged and not removed from the vicinity of the pulsing SGC will quickly lead to a temperature rise and the loss of superconductivity. AC losses in practical superconductors are generated by three main mechanisms: 1) hysteresis, 2) eddy current, and 3) coupling. Hysteresis losses can be reduced by using wires with fine superconducting filaments (i.e. multifilamentary wires.) Eddy current losses can be reduced by decreasing the length of the current paths through the normal conductivity materials resident in the cross-section of the superconductive wire. Coupling losses can be reduced by using a matrix of relatively high resistivity material (e.g. Cu—Ni or Cu—Sn (bronze)) between the superconducting filaments, and by twisting the wire as tightly as possible. Therefore a superconducting wire for an AC application would have fine filaments, preferably less than 10 micro-meter in thickness, would be twisted, preferably with a twist pitch tighter than 1 turn per 5 cm, would have inter-filament material matrix that has high resistivity, and copper stabilizer that is configured to reduce eddy current paths. A feasible conductor for an AC superconducting magnet might be a cable of relatively fine superconducting wires with attributes described above. A cable composed of fine wires: a) allows tighter twisting of the individual wires, b) creates relatively shorter eddy current paths because of the smaller diameter, c) facilitates the wire manufacturing process for creating fine superconducting filaments, and d) increases the effective twist pitch because after twisting the individual wires, the overall cable is twisted as well.
Practical engineering (both universal and application-specific) problems associated with the substitution of SGC-based systems for copper gradient coil systems remain. A universal one is that since losses inherent in the generation of time-varying fields are inevitable, the resultant heat generated from such losses must, with adequate thermal management, be removed from the SGC region. This universal problem along with other issues associated with, for example, the detrimental coupling of the time varying fields of an SGC with instruments in its vicinity (e.g. the so-called B0 DC superconducting magnet within which the SGC system may be required to operate) are addressed in this disclosure.